1. Mechanics of Flow Control
EE382C Lecture 8
Flow Control Part 2
Virtual Channels
Mechanics of Flow Control
Deadlock
4/21/11
EE 382C - S11- Lecture 8

2. Logistics HW2 due today HW3 out today - see next slide - due 4/28
Midterm IN CLASS on 5/3
EE 382C - S11- Lecture 8

3. Homework 3
Problems 13.3, 14.2, 14.9
EE 382C - S11- Lecture 8

4. Pending Question from Tuesday
If storage is plentiful and channel bandwidth is expensive, what flow-control strategy should you use?
Hint: Does this depend on packet length?
EE 382C - S11 - Lecture 7

5. Question of the day
What is the minimum number of virtual channels needed to break deadlock in a mesh network using virtual-channel flow control?
EE 382C - S11- Lecture 8

6. Circuit Switching (II)
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7. Flow Control Review Allocate resources To units of information
Channel bandwidth, buffer space, router state
To units of information
Packets or Flits
Unbuffered flow control
Circuit switching
Dropping
Misrouting
Buffers decouple channel allocation
Packet-Buffer Flow Control
Store-&-Forward
Virtual cut-through
Flit-Buffer Flow Control
Wormhole
Virtual Channel
Virtual channels decouple channel dependencies
EE 382C - S11- Lecture 8

8. Buffered Flow Control Packet-Buffer FC Flit-Buffer FC
Store and Forward
Virtual Cut Through
Flit-Buffer FC
Wormhole
Virtual Channels
Why buffer?
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9. Packet-Buffer Flow Control
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10. Store-and-Forward
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11. Virtual Cut-through
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12. Packet-Buffer Flow Control
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13. Allocation units
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14. What is good about packet buffers? What is bad about packet buffers?
Don’t idle channels when a packet is partly transmitted and you can’t get a buffer
Of course you may idle a channel because you can’t get a buffer to start sending.
Two main issues:
They are big – this increases cost and energy
Not the most efficient way to use a fixed amount of buffer space
Big buffers soften backpressure – you can send a lot of flits the wrong way before you sense congestion.
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15. Wormhole The Movie
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16. Head flit arrives, allocate VC state, request upper output
W is waiting state
U is upper output channel
L refers to lower input channel
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17. Body flit arrives, still waiting on upper output
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18. Output allocated, forward head flit, second body flit arrives
A is active
Both ports now select upper U
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19. Tail arrives, body flit forwarded
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20. Second body flit forwarded
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21. Tail flit forwarded, input and output VC states deallocated
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22. Virtual Channel Flow Control
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23. Blocking of wormhole flow control
Example of inefficiency – channel is idle even though a packet could use it
Why?
Because blocked packet owns the buffer
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24. Virtual-channel flow control decouples dependency between buffer and channel
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25. Sometimes, its better to be unfair (part 1)
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26. Fair arbitration interleaves flits. Both packets are impeded
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27. Winner-take-all arbitration impedes only one packet – by the same amount as interleaving
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28. Q When should you use winner-take-all? And when shouldn’t you?
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29. Is “Winner Take All” just Wormhole?
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30. State of a VC router
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31. Wide vs long buffers (Which of these is better?)
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32. Many small buffers wins non-interference with stiff backpressure
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33. VCs as a Swiss Army knife
EE 382C - S11- Lecture 8

34. Pending Question from Tuesday
If storage is plentiful and channel bandwidth is expensive, what flow-control strategy should you use?
Hint: Does this depend on packet length?
EE 382C - S11 - Lecture 7

35. Mechanics of Flow Control
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36. Credit-Based Flow Control
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37. On/Off Flow Control
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38. Ack-Nack Flow Control
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39. Deadlock
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40. Definition
Deadlock
A condition in which a set of agents waits indefinitely trying to acquire a set of resources
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41. Examples Dining Philosophers Circuit Switching Who are the agents?
What are the resources?
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42. Deadlocked Configuration
Waits-for, held-by graph
Circuit switching example
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43. Example
Circuit-switched ring network
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44. Resource Dependence
A resource A is dependent on a resource B if it is possible for A to be held-by an agent X and it is also possible for X to wait-for B
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45. Cyclic Resource Dependence
A cycle in the resource dependence graph means that a deadlocked configuration is possible.
It does not mean that deadlock has occurred or will occur.
EE 382C - S11- Lecture 8

46. Example Consider the same 4-node ring with packet-buffer flow control
What is the dependence graph?
What is the held-by, wait-for graph for a deadlocked configuration?
How can you avoid deadlock in this network?
EE 382C - S11- Lecture 8

47. Example
Consider the same 4-node ring network with virtual channel flow control
What is the dependence graph?
How can you avoid deadlock in this network?
EE 382C - S11- Lecture 8

48. Protocol Deadlock
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49. Next Time Dealing with Deadlock Two main approaches
Just say NO! (to deadlock)
AKA Deadlock Avoidance
Prevent a deadlock from ever occurring
e.g., by resource ordering
Fix it
AKA Deadlock Recovery
Detect a deadlock and correct it
e.g., by dropping packets or using “escape paths”
EE 382C - S11- Lecture 8

50. Question of the day
What is the minimum number of virtual channels needed to break deadlock in a mesh network using virtual-channel flow control?
EE 382C - S11- Lecture 8

51. Summary Virtual Channel Flow Control Flow Control Mechanics Deadlock
Partition channel state
Allow packets to “pass” blocked packets
Many uses
Performance
QoS
Non-interference
Deadlock avoidance
Flow Control Mechanics
Backpressure requires signaling
Credits, on/off, ack/nack
Deadlock
Occurs when agents “wait forever” for resources
Agents: connections, packets
Resources: channels, buffers, virtual channels
Deadlocked configuration
Resource dependence
EE 382C - S11- Lecture 8